The invention relates to an integrated semiconductor arrangement comprising an active isolator circuit that includes at least one field effect transistor.
The invention finds its application in the realization of microwave frequency systems such as an antenna front end for satellites in which it is necessary to isolate the local oscillator from impedance variations of the mixer. The invention also finds its application in the realization of microwave circulators.
An isolator circuit is known from both U.S. Pat. No. 4,679,010, July 1987, and the publication entitled: "The design of a 6-Port active circulator" by I. J. BAHL in 1988 IEEE, MTT-S Digest, pp. 1011-1014. These documents describe an active circulator circuit constituted by three isolator circuits and three couplers.
More particularly the publication points out that in many microwave circuits non-reciprocal components such as isolators or circulators are useful for avoiding interaction between the various blocks of the system. In the conventional microwave systems a ferrite material was used to realize such components. At present, these components can be realized on GaAs substrates by adding ferrite disks and permanent magnets. However, this technique results in a very large size and considerable weight of the components and is not compatible with monolithic microwave integrated circuit technology. Therefore, the above-mentioned Patent and publication propose a completely integrable active circulator. For this purpose, in the aforementioned publication the isolator circuits necessary for realizing the circulator are constituted by monolithic distributed amplifiers having four cells operating between 1 to 10 GHz with a gain of 7 dB, return losses and isolation exceeding 12 dB and 20 dB respectively. Such an amplifier uses 4 field effect transistors having a gate of 0.5.times.150 .mu.m and dissipates about 60 mW. A complete description of these isolator circuits can be found in the publication entitled: "A Monolithic GaAs 1-13 GHz Travelling-Wave Amplifier" by YALCIN AYASLI et al. in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-30, No. 7, July 1982, pp. 976-981.
This prior art isolator circuit presents various disadvantages:
the 12-dB return losses and the 20-dB isolation realized by means of this circuit are not sufficient for the considered application mentioned above;
the use of 4 transistors to form this circuit leads to a considerable power consumption of 60 mW;
the occupied surface is large (2 mm.sup.2) which presents a disadvantage as to its use in integrated circuits;
the performance is sensitive to the technological parameters.
In contradistinction, the isolator circuit of the above-mentioned Patent is different.
In the above Patent the isolator circuit comprises a common-gate FET at the input and a common-drain FET at the output. The size of the FET's is optimized to obtain a good adaptation to 50 Ohms.
The isolator known from this Patent further includes series L-C circuits, connected between the transmission line and ground whose function is to balance the isolator. The first transistor operates as an amplifier arranged for the high frequencies. The circuit is provided to block the low frequencies in either one of the two directions.
This above-mentioned Patent points out that the second transistor presents a gate-source impedance which is so high that a reflected signal, entering the output terminal and propagating in the reverse input-output direction, has an attenuation of 25 dB.
In reality those skilled in the art know that the gate-source capacitance of a transistor is approximately 10 times smaller than its gate-drain capacitance. The performance disclosed in the above Patent therefore seems utterly overestimated.
Another publication entitled "ACTIVE ISOLATOR, COMBINER, DIVIDER, AND MAGIC-T AS MINIATURIZED FUNCTION BLOCKS" by Tsuneo Tokumitsu, in GaAs IC SYMPOSIUM IEEE 1988 pp. 273-276 describes the same isolator circuit.
In this publication it is described that the isolation is only 20 dB at 10 GHz with input return losses of 15 dB at the same frequency.
This performance is insufficient for the applications contemplated for the present invention.
On the other hand, the isolator circuits known from the Patent and from the latter of the above publications comprise 4 inductive impedances which are elements each requiring a large surface in a circuit, and which are thus to be avoided as much as possible for the applications in integrated circuits.